Video display systems wherein information is stored on a substantially transparent disc are well known in the prior art, for example, see Bouwhuis U.S. Pat. No. 3,855,426; Gregg U.S. Pat. No. 3,430,966; Feinlieb U.S. Pat. No. 3,626,336; and Johnson U.S. Pat. No. 3,518,442. While these patents describe focusing video systems wherein a light source is focused on that part of the video disc where the subject information is recorded, imaging video display systems are also known in the prior art. See Wohlmut et al U.S. Pat. Nos. 3,848,095 and 3,946,367, the disclosures of which are incorporated herein by reference.
In imaging video systems, a light source floods an area of the disc, rather than being focused on a particular portion thereof. Light passing through the disc is modulated in accordance with the information stored thereon. This modulated light forms an image of the track itself and is focused by an objective lens onto a photosensitive detector. The photosensitive detector produces an output responsive to the modulated light impinging thereon, which is therefore suitable for use an an input to conventional display circuitry. While these prior art video systems have functioned well, certain difficulties have existed resulting in less than optimal reproduction of the stored information.
One of these difficulties has been unavoidable eccentricities of the data track on the disc during playback. Due to the mechanical means used to drill spindle holes, and also nonuniformities in the disc material, the spindle hole may be positioned away from the geometric center of the disc. As a result, fluctuations in data track radius of up to 125 microns may occur. Also, simple nonuniformities in the disc material may cause the data track spacing to vary; mechanical vibrations may also cause unwanted variation. Since the track spacing on the discs is typically 2.2 microns center-to-center, and the tracks themselves are typically 1.1 microns wide, such miscentering can result in an error of greater than one track. Therefore, it is desirable for the detection system of the playback unit to tolerate eccentricities in track radius of at least 125 microns. While photosensitive devices have been used to detect changes in a slowly varying medium, see for example Desai U.S. Pat. No. 3,609,373, such devices have not been applied to the present problem of following a fluctuating, rapidly rotating data track.
Another difficulty which has occurred relates to actual readout of the track information. A typical system may use mirrors and galvanometers to determine the location of the selected data, to permit data readout. The mechanical limitations of such devices inherently result is some misalignment which in turn degrades the ultimate information display. Furthermore, the response time of such mechanical devices is generally too slow to accurately monitor a rapidly varying medium. It is therefore useful to provide a system not limited by the slow response times and misalignment inherent in mechanical systems.
Also, prior art systems have typically been limited to readout of the information stored on a single segment of a spiral track covering the disc. For various applications, specifically video games, message scramblers, digital information and video graphics, it is desirable to simultaneously read out information contained on parallel tracks. Therefore, it is desirable to provide a system wherein parallel simultaneous and parallel sequential read modes are possible.
Another difficulty with the prior art has been nonuniformity in rotational velocity of the video disc. Since the information on the video track is spaced at regular intervals, accurate reproduction of the stored information requires data flow to be maintained at a constant rate. With a video disc system, this requires a constant rotational velocity. Slight fluctuations in disc velocity are unavoidable, thus the fluctuations in disc velocity must be compensated for to provide the requisite uniform information flow. While systems for providing time base correction exist in the prior art, such systems are prohibitively expensive for a commercially practical system. Therefore it is desirable to provide a system which can economically compensate for such variations in track velocity.
Poor signal-to-noise ratio during playback has been another cause of degraded video signal in prior art devices. The prior art systems typically use a single detector or occasionally a pair of detectors to transform the variations in transmitted light representing the information stored on the data track into usable electrical signals. With such a small number of detectors, the information readout is susceptible to various noise sources such as fluctuations in light source intensity. Since only a small number of detectors are used in the prior art this source of noise is reflected directly in the video display. While improving the signal-to-noise ratio in a system having only a small number of detectors is possible by using complex and expensive circuitry, such a system is economically infeasible. It is therefore desirable to provide an improved detector system wherein an improved signal-to-noise ratio is provided.
A further difficulty frequently found with display of video information is that the information is so densely packed that occasional imperfections in the disc or debris on the disc cause the quality of the stored information to deteriorate. For example, sometimes the presence of dust or scratches will entirely obliterate a portion of the information to be played back. It is therefore desirable to provide a method and means for reproducing such information in spite of such debris and imperfections.
Another difficulty with the prior art has been degradation in frequency response. Frequency response can generally be regarded as indicated by the sharpness of the sync pulse which begins a time of video information. Because of inherent optical and mechanical limitations, a sync pulse which is ideally a square pulse in reality exhibits some round-off. This round-off in turn indicates some loss of frequency response, especially at higher frequencies such as two megahertz and above. Since the bandwidth of black and white information may exceed two megahertz, and color information requires a bandwidth of 3.58 megahertz, noticeable degradation of image quality occurs with even small amounts of rounding-off of the sync pulse. Thus a system which can compensate for losses in frequency response is desirable.
Poor focusing has been a problem with certain other prior art systems. Loss of focus can result in a degraded color or other reference signal, as well as other signal degradation, which causes a proportionate decrease in information quality. The typical prior art systems have involved opto-mechanical systems or systems for oscillating a lens over a slight distance to provide good focusing. However, such systems are typically not economically suitable for maintaining an optimum focus at high data packing, where good focus becomes critical.
One object of the present invention is to provide an improved means for following the data track of a disc despite eccentricities in the disc.
Another object of the present invention is to provide an improved means for readout of track information.
Another object of the present invention is to provide for parallel simultaneous and sequential readout of stored data.
Another and further object of the present invention is to compensate for variations in rotational velocity of a storage disc.
Another object of the present invention is to provide means for improving the signal-to-noise ratio of stored information during playback.
Another object of the present invention is to provide means for dropout compensation where information stored at a high packing density is lost.
Another object of the present invention is to provide means for enhancing the frequency response of the storage medium.
Other and further objects of the present invention will be apparent from the subsequent detailed description.